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What are the components of an atom? How much does each atom weigh?

The answer to both your questions are tucked away in one of coolest charts anyone has ever thought up. Around 100 years ago this very organized sort of guy named Dimitri Ivanovich Mendeleyev put the names and properties of all the chemicals he knew onto cards. He then tried arranging them in a way that made sense in a sort of Chemist's game of Solitaire. The resulting chart, called The Periodic Table of Elements, has contributed probably more than anything else to our understanding of matter. When you begin studying the Periodic table in school, pay close attention. There is a tremendous amount of information and beauty tucked away on that single sheet if you understand the subtleties.

Ok, how does that answer your questions? The first question is easy. Atoms being made up of combinations of protons, electrons and usually, but not always, some neutrons thrown in for fun. Take a look at our tour of the atom for details.

The answer to the second question is, "It depends on what atom it is." The weight of each element is different depending on the number of protons and neutrons it has. The elements in Mr. Mendeleyev's list are arranged by their atomic number, which is the number of protons in that element. Also listed is that elements atomic weight, which is really the average number of protons and neutrons. I hope you noticed I said average weight, because elements often exist in different varieties called isotopes. This means that they have a different number of neutrons. The atomic weight listed is the average of the atomic weights of the element and its isotopes. If the number of protons was different it would be a different element. The mass of the electrons is very small and ignored in all this. The number of electrons is the same as the number of protons, but all sorts of things in nature can change that. The unit of atomic weight was fixed at 1/12 the atomic weight of a carbon atom.

Now it gets tricky. The atomic weight times a constant called Avogadro's number (6.0225*1023) is the amount of atoms that have a mass in grams equivalent to the atomic weight. Let's pick an example of the third most common element in the universe, carbon, with an atomic weight of 12.01. That means 12.01 grams of carbon contains 6.0225*1023 atoms of carbon. If you divide the atomic weight by Avogadro's number, you get the mass of one atom. In the case of carbon, that ends up being 0.000000000000000000000019942 grams. Try weighing that on a bathroom scale!


Brian Kross, Chief Detector Engineer (Other answers by Brian Kross)

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